Divide the RPM task into two smaller tasks to make SetupBuilder more flexible

Hi @gamma ,
me again.
I have a question.
Could the rpm task be divided into two smaller tasks?
The `SPEC file generate task` and the `rpm generate task`?
So that if I have some special requirement for the rpm that the SetupBuilder does not support, I can call the `SPEC file generate task` first, then do some replacement in the SPEC file, at last call the `rpm generate task`.

Start by locating the RPM task implementation in SetupBuilder and trace how it currently generates the SPEC file and RPM. Done means the workflow exposes separate SPEC-file generation and RPM-generation tasks, allowing edits between them while preserving the existing RPM build behavior.

Written by the indexing model from the issue text.